This invention relates to lithium metal oxide positive electrodes for non-aqueous lithium cells and batteries. More specifically, it relates to lithium-metal-oxide electrode compositions and structures, having in their initial state in an electrochemical cell, a general formula xLiMO2.(1-x)Li2Mxe2x80x2O3 alternatively Li2-xMxMxe2x80x21-xO3-x in which 0 less than x less than 1 and where M is one or more trivalent ion with at least one ion being Mn, and where M is one or more tetravalent ions selected preferably from Mn, Ti and Zr; or, where M is one or more trivalent ion with at least one ion being Ni, and where Mxe2x80x2 is one or more tetravalent ion with at least one ion being Mn. In one embodiment of the invention, the Mn content should be as high as possible, such that the LiMO2 component is essentially LiMnO2 modified in accordance with this invention. In a further embodiment of the invention, the transition metal ions and lithium ions may be partially replaced by minor concentrations of one or more mono- or multivalent cations such as H+ derived from the electrolyte by ion-exchange with Li+ ions, and/or Mg2+ and Al3+ to impart improved structural stability or electronic conductivity to the electrode during electrochemical cycling.
Lithium-metal oxide compounds of general formula LiMO2, where M is a trivalent transition metal cation Co, Ni, Mn, Ti, V, Fe, and with electrochemically inactive substituents such as Al are very well known and are of interest as positive electrodes for rechargeable lithium batteries. The best-known electrode material is LiCoO2, which has a layered-type structure and is relatively expensive compared to the isostructural nickel and manganese-based compounds. Efforts are therefore being made to develop less costly electrodes, for example, by partially substituting the cobalt ions within LiCoO2 by nickel, such as in LiNi0.8Co0.2O2 or by exploiting the manganese-based system LiMnO2. Such layered compounds are sometimes stabilized by partially replacing the transition metal cations within the layers by other metal cations, either alone or in combination. For example, Mg2+ ions may be introduced into the structure to improve the electronic conductivity of the electrode, or Al3+ or Ti4+ ions to improve the structural stability of the electrode at high levels of delithiation. Examples of such compounds are LiNi0.8Co0.15Al0.05O2 and LiNi0.75Co0.15Ti0.05Mg0.05O2.
A major problem of layered LiMO2 compounds containing either Co or Ni (or both) is that the trivalent transition metal cations, M, are oxidized during charge of the cells to a metastable tetravalent oxidation state. Such compounds are highly oxidizing materials and can react with the electrolyte or release oxygen. These electrode materials can, therefore, suffer from structural instability in charged cells when, for example, more than 50% of the lithium is extracted from their structures; they require stabilization to combat such chemical degradation.
Although the layered manganese compound LiMnO2 has been successfully synthesized in the laboratory, it has been found that delithiation of the structure and subsequent cycling of the LixMnO2 electrode in electrochemical cells causes a transition from the layered MnO2 configuration to the configuration of a spinel-type [Mn2]O4 structure. This transformation changes the voltage profile of the Li/LixMnO2 cell such that it delivers capacity over both a 4V and a 3V plateau; cycling over the 3V plateau is not fully reversible which leads to capacity fade of the cell over long-term cycling. Other types of LiMnO2 structures exist, such as the orthorhombic-form, designated O-LiMnO2 in which sheets of MnO6 octahedra are staggered in zigxe2x80x94zig fashion unlike their arrangement in layered LiMnO2. However, O-LiMnO2 behaves in a similar way to layered LiMnO2 in lithium cells; it also converts to a spinel-like structure on electrochemical cycling.
Therefore, further improvements must be made to LiMO2 electrodes, particularly LiMnO2, to impart greater structural stability to these electrode materials during electrochemical cycling in lithium cells and batteries. This invention addresses the stability of LiMO2 electrode structures, particularly LiMnO2, and makes use of a Li2Mxe2x80x2O3 component to improve their stability.